1. Field of the Invention
The invention relates to hybrid rings, that is, hybrid junctions consisting of a waveguide or transmission line forming a closed ring into which lead four guides or lines appropriately spaced around the circle. In particular the invention relates to a hybrid ring in which the ring impedances are a function of both load impedances at input and output of the device as well as the power division ratio at the two output ports of the hybrid ring.
2. Prior Art
Hybrid rings are well known in the prior art and are defined in ANSI/IEEE Std 100-1977 American National Standard, approved May 12, 1978, American National Standards Institute, wherein the definition of a hybrid junction may also be found set forth as, "a waveguide or transmission line arrangement with four ports which, when the ports have reflectionless terminations, has the property that energy entering any one port is transferred (usually equally) to two of the remaining three." References cited frequently with regard to hybrid ring structures are U.S. Pat. No. 2,445,895, to W. A. Tyrrell, issued July 27, 1978, as well as Tyrrell's paper entitled "Hybrid Circuits for Microwaves", published in the November 1947 issue of the Proceedings of the IRE.
W. D. Lewis in U.S. Pat. No. 2,639,325, issued May 19, 1953, notes that an inconvenient feature of the prior art hybrid rings (referring particularly to FIGS. 12 and 37 of the Tyrrell patent) is the fact that the impedances required for the four circuits to be coupled to the four terminals of the hybrid ring structure, respectively, differ from terminal to terminal. Lewis then discloses a hybrid ring in which the ring impedance is uniform and the four output terminals are matched to a single load impedance. He achieves this end by spacing the four terminal ports around the ring structure so as to obtain a match between the port load impedances and the uniform impedance of the ring.
By its very nature, the hybrid ring is a frequency sensitive device. This is true because its proper functioning is dependent upon the electrical path length about the ring structure as well as the length of the electrical path separating the four ports coupled to the ring structure. Those skilled in the art have been active in attempting to increase the effective operating bandwidth of hybrid ring devices.
The ring hybrid depends in general upon a ring structure whose electrical and physical path length are each equal to one and one-half wavelengths at the design frequency. Hylas in U.S. Pat. No. 2,735,986, issued Feb. 21, 1956 provides a broad band hybrid ring network by reducing the physical path length of the ring to one wavelength at the design frequency while maintaining the electrical path length about the ring structure at the required one and one-half wavelengths. This is accomplished by structuring the ring of a two conductor transmission line and transposing the conductors at a point between two of the terminals connected to the ring structure. This transposition of conductors effectively introduces a frequency insensitive 180.degree. phase shift. Such frequency insensitivity naturally increases the operating bandwidth of the device. Hylas makes further improvements in the effective bandwidth performance of the device by impedance matching at the junction at which the ports are coupled to the ring structure.
Cappucci in U.S. Pat. No. 3,504,304, issued Mar. 31, 1970, characterizes prior art hybrids such as those disclosed by Hylas as " . . . devices which provide the required isolation between conjugate junctions only over a relatively narrow frequency band of signals applied to the input." Cappucci then discloses a hybrid ring which utilizes the conductor transposition of Hylas but further includes compensating circuits having the reactive portion of their impedances variable between inductive and capacitive reactances over the operating range of the hybrid ring. This is accomplished by the use of a series resonant circuit connected to each of the four junctions of the ring structure. The effect is stated as increasing the operating bandwidth and/or decreasing the input voltage standing wave ratio (VSWR).
Budenbom has several patents concerning the broadband operation of hybrid rings. In U.S. Pat. No. 2,784,381, issued Mar. 5, 1957 hybrid structures having greater than four arms are disclosed in a coupling arrangement stated to yield an increased useful frequency range of operation. A hybrid ring having a given number of branch taps or arms is connected in tandem with two or more hybrid rings having a greater number of branch arms or taps in such a manner as to merely add logrithmically the attenuations obtainable between conjugate taps or arms of the several hybrid ring structures. In U.S. Pat. No. 3,010,081, issued Nov. 21, 1961, there is disclosed two similar four-arm hybrid rings connected in parallel, with the connections to one ring having a mirror image relationship with respect to the connections of the other. The output of the two hybrid rings is combined in a third ring. It is stated that the frequency range over which the balance is high is greatly increased because an unbalanced voltage developed in one ring, as the frequency is changed, is cancelled by an equal unbalanced voltage of opposite polarity developed in the other ring. In U.S. Pat. No. 2,959,751, issued Nov. 8, 1960, phase compensation is provided to offset the frequency sensitivity of the path lengths within the ring structure. The phase compensation is to provide an essentially frequency insensitive half-wavelength path difference in the two path lengths between input port and difference output port.
A promising phase reversal network has been diclosed by Steven March, in a paper entitled, "A Wide Band Strip Line Hybrid Ring", IEEE Trans., Volume MTT-16, page 361, (June 1968). March replaces the three quarter wavelength line section of the conventional hybrid ring with a pair of equilaterial, broad side coupled, quarter wavelength segments of transmission line having a pair of diameterically opposed ends short circuited. This quarter wavelength network provides phase reversal over a wide frequency band. Use of such a phase reversing network reduces the overall size of the hybrid ring structure.
Size has always been a drawback in the use of hybrid ring structures. This is further complicated by the necessity of providing transformer networks to match the impedance of such devices as transistors, Gunn diode amplifiers and oscillators depending upon the choice of device employed with the hybrid ring port loading impedances may have to be matched to impedances in the 5 to 100 ohm range. The necessity of providing transformer networks between the hybrid ring and such active devices generally increases the overall length, weight, and cost of the package and increases insertion loss of the overall device.
It is therefore seen that an unfulfilled need exists for a hybrid ring network which will inherently perform the necessary impedance transformation to match the hybrid ring and the active devices associated with it. A branch-line hybrid having such inherent impedance transformation characteristics has been disclosed by Chen Y. Ho in his paper, "Transform Impedance With a Branch-Line Coupler", Microwaves, Volume 15, pages 47-52, (May 1976). Application of Ho's approach however produces a narrow bandwidth device (approximately 10 percent). For wider bandwidth operation, those skilled in the art at this present time must resort to the conventional use of external transformer matching networks and a broader bandwidth coupler such as a ring hybrid coupler with its 26 percent bandwidth or the coupled-line coupler with its octave bandwidth capabilities.
It is therefore an objective of the invention to provide a hybrid ring having inherent impedance transformation characteristics to permit matching of the impedance of the ring structure to that of an external device.
It is a further objective of the invention to provide a hybrid ring structure having inherent impedance transformation characteristics capable of matching the ring structure to external load impedances wherein the input port load impedances differ from the output port load impedances.
It is another objective of the invention to provide a hybrid ring structure with inherent impedance transformation characteristics having a useful operating bandwidth at least equivalent to that of prior art non-impedance-transforming hybrid rings.
It is a more particular objective of the invention to provide a hybrid ring having inherent impedance transforming characteristics which is capable of useful operation over octave bandwidths.
It is a specific object of the invention to provide an impedance transforming hybrid ring wherein the ring impedances are established as a function of both input and output load impedance characteristics as well as of the power division ratio at the two output ports of the hydrid ring.